Fluorescence is a well-known method that, with the aid of tuned filters, conveys a defined excitation spectrum to a specimen, spectrally separates the response signal radiated by the specimen from the excitation light, and passes that signal on for observation and analysis. In the clinical field, for example, many applications are known which assist surgical operations and which mark, by way of the emitted fluorescence, the tissue that is to be resected. One particular example of the application of such a method using fluorescence devices integrated into a microscope are surgical microscopes for neurosurgery, which use photodynamic medications, known e.g. under the names aminolevulinic acid (ALA) or meso-tetrahydroxyphenyl chlorine (mTHPC), to permit more complete excision of certain tumors.
Another application relates, for example, to infrared angiography, in which light from the near-infrared (NIR) region is used for excitation, in order then to observe the specimen in the longer-wavelength spectral region. Other applications make use of invisible ultraviolet light. Other spectral regions, from ultraviolet to blue light and from there to red and on into the far infrared, are likewise possible.
At the point where a tissue or a specimen needs to be excited, a sufficiently intense excitation spectrum is of essential importance. When working, for example, with blue excitation light in the range from 380 to 420 nm, a specific fluorescence signal (e.g. 635 nm with ALA) will be obtained depending on the fluorescence ingredient that is used. 300-watt xenon light sources are generally used for this; they make available both normal microscopy white light and the blue light necessary for fluorescence, the latter by filtering and optimizing the spectral region from 380 to 420 and by careful selection of the xenon element. The same analogously applies, of course, to other spectral regions. Examples of such known microscopes or surgical microscopes are found, for example, in U.S. Pat. No. 6,510,338 or DE-A-195 48 913, in which the light of the illumination device is conveyed to the specimen being observed via optical waveguides and other optical devices.
A problem with these known microscopes is that in selecting the illumination source, a compromise must be made whose ultimate result is that the white-light quality cannot be optimized for observation, and that on the other hand, specifically when the blue-light component is enhanced and optimized, other spectral regions are underrepresented and then lead to color casts in the standard white light situation. The color cast can theoretically be corrected using filters, but that then also causes a reduction in intensity. On the other hand, true-color observation of a surgical field is important not least for diagnostic purposes. It is not possible to raise the intensity by way of an increase in lamp output, however, because of the limited aperture of the microscope's illumination optics as well as other effects.